Intestinal infections with protozoan parasites such as Entamoeba histolytica, Giardia lamblia, Cryptosporidia, and Isospora belli are frequently found in homosexual men, individuals living in tropical areas, and travelers. Organisms such as Cryptosporidia and Isospora belli are being found high prevalence rates in tropical countries and in patients with the acquired immune deficiency syndrome (AIDS). Enzyme linken immunosorbent assays (ELISA) for the detection of pathogenic antigen in stool specimens offer the advantage of being rapid, easy to perform on large numbers of specimens, standardized without difficulty and adjunctive in epidemiologic studies. We have recently developed indirect double antibody ELISA systems to detect G. lamblia and E. histolytica in single stool specimens. Homosexual men at risk for AIDS have recently been screened for these parasites with the above assays. For Giardia, stool specimens were positive by ELISA in 31 of 35 (88%) of patients with giardiasis by multiple stool exams; negative specimens came from patients with low number of parasites in the stool. In 10 patients followed prospectively, stools became negative by ELISA after successful treatment. Stool specimens were positive by ELISA in only 9 of 220 (4%) of patients without demonstrable giardia in their stool. For E. histolytica, 57 of 62 (92%) patients infected with E. histolytica were positive by ELISA. Five of 193 (3%) from patients without demonstrable E. histolytica in their stool were positive by ELISA. Coinfection with multiple other parasites did not appear to affect the sensitivity or specificity of these tests. An ELISA for the adequate detection for Cryptosporidia or Isospora has so far not been possible, utilizing a variety of different antibody assays. Rapid diagnosis of these pathogens has been accomplished by the auramine/acid fast stain. This stain has enabled us to detect these two pathogens in 4% and 1% respectively of the homosexual population which was screened. The significance of this project is to utilize rapid diagnostic tests for intestinal protozoans to detect infection and carriage of these protozoans in a high risk population for AIDS. Future aspects of this project will entail the screening of patients living in endemic areas such as Zaire and correlation with epidemiologic, immunologic and clinical features of the intestinal disease caused by these pathogens.